1. Field of the Invention
The present invention relates to a method for producing a molded product, a method for producing a molded product having a plating film, a method for producing a resin pellet, and a foam molded product having a plating film. Further, the present invention relates to a foam injection molding method.
2. Description of the Related Art
As a method for forming a metal film on a molded product at low cost, there has been known an electroless plating method. In the electroless plating method, in order to ensure tight contact performance of the metal film to the molded product, it is performed a pretreatment in which the surface of the molded product is roughened by using an etching solution including an oxidizing agent such as hexavalent chromic acid, permanganic acid, or the like. Thus, ABS resin (acrylonitrile-butadiene-styrene copolymer synthetic resin) eroded by the etching solution has been mainly used in the electroless plating method. Butadiene rubber component of the ABS resin is selectively eroded by the etching solution, thereby forming concavity and convexity on the surface of the ABS resin. On the other hand, regarding a resin other than the ABS resin such as polycarbonate or the like, plating grade, which is mixed with the component, such as the ABS resin, elastomer, or the like, selectively oxidized by the etching solution, is commercially available so as to enable the electroless plating. However, the pretreatment for the electroless plating method has a problem such that environmental burden is heavy, because hexavalent chromic acid, permanganic acid, or the like is used.
Meanwhile, as a method for forming the metal film on the molded product without the etching step as the pretreatment, there has been suggested use of a surface modification method of the molded product in which pressurized carbon dioxide such as supercritical carbon dioxide is used. The inventors of the present invention have suggested a method in which the surface modification process using pressurized carbon dioxide is performed along with injection molding to disperse metallic fine particles, such as palladium, which function as the catalyst cores of the electroless plating, on the surface of the molded product (Japanese Patent Application Laid-open No. 2005-280362, Japanese Patent Application Laid-open No. 2010-30106 and Japanese Patent No. 4160623). In this method, by performing the electroless plating with respect to the surface of the molded product including the metallic fine particles, it is possible to form a plating film on the surface of the molded product without the etching step.
Further, Japanese Patent Application Laid-open No. 2007-130982 suggests as follows. That is, another functional material is used, instead of the metallic fine particles, in the surface modification method of the molded product using pressurized carbon dioxide such as supercritical carbon dioxide, thereby modifying the surface of the molded product to have highly advanced function property or high performance property.
Further, in recent years, a foam injection molding method using a physical foaming agent such as supercritical nitrogen, supercritical carbon dioxide, or the like has been studied and practically used (for example, Japanese Patent No. 2625576). This method is, for example, a technique or approach as follows. At first, the physical foaming agent is brought into contact with a resin plasticized and melted in a hermetically closed plasticizing cylinder, and the physical foaming agent and the resin plasticized and melted are compatibly dissolved with each other. Next, the molten resin in which the physical foaming agent is dissolved is weighed, while maintaining the high pressure in the plasticizing cylinder to such an extent that the physical foaming agent can be in a supercritical state, and the molten resin is injected and charged into a mold. The supercritical fluid compatibly dissolved in the molten resin upon the injection and charging is subjected to sudden pressure reduction and gasified. By solidifying the molten resin, foams (bubbles) are formed at the inside of the molded product.
The foam injection molding using the supercritical fluid is clean and any residue of the foaming agent does not remain as compared with foam molding using any chemical foaming agent. Further, since the foamed cell diameters become fine and minute, mechanical strength of the molded product is hardly lowered. Furthermore, since the high pressure physical foaming agent functions as a plasticizing agent for the molten resin, the following advantages are also provided. That is, resin viscosity is decreased upon the injection and charging and the fluidity is improved. Sink marks are suppressed, which would be otherwise caused by the shrinkage when the resin is solidified on account of the gas pressure upon the foaming. The latent heat is deprived from the interior of the molten resin upon the foaming and thus the cooling strain and the warpage are decreased.
As shown in FIG. 15, a general injection molding apparatus 4000 using the physical foaming agent such as the supercritical fluid includes a bomb 922 for supplying the physical foaming agent such as nitrogen, carbon dioxide, or the like, a high-pressure device 918 for raising pressure of the physical foaming agent and controlling the feed of the physical foaming agent, and a plasticizing cylinder 907 in which the resin is plasticized and melted and the resin is mixed with the physical foaming agent. The physical foaming agent is fed from the bomb 922 to the high-pressure device 918, and the pressure of the physical foaming agent is raised by the high-pressure device 918. Then, the physical foaming agent is introduced into the plasticizing cylinder 907 by an introducing valve 920, which is opened and closed intermittently, via an injection adjustment mechanism 917. The introduction amount of the physical foaming agent into the plasticizing cylinder 907 is, for example, adjusted by the following method. The physical foaming agent always circulates between the high-pressure device 918 and the injection adjustment mechanism 917. The pressure of a circulation system is higher than the pressure in the plasticizing cylinder 907 and a constant differential pressure is held with respect to the pressure in the plasticizing cylinder 907. The flow rate of the circulation system is constantly controlled by an orifice or the like incorporated into the high-pressure device 918. Then, the physical foaming agent is taken from the circulation system by opening and closing of the introducing valve 920 and introduced into the plasticizing cylinder 907. The introduction amount of the physical foaming agent is controlled by an open time of the introducing valve 920 and the like, based on the flow rate of the circulation system etc.
As described above, in order to provide the constant flow rate of the physical foaming agent to be supplied to the plasticizing cylinder 907, the system of the high-pressure device 918 in the conventional injection molding apparatus 4000 has complexity. Further, in order to obtain the stable flow rate of the physical foaming agent, it is necessary to prepare the physical foaming agent having pressure several times higher than the introduction pressure (for example, 30 to 40 MPa). Therefore, the injection molding apparatus 4000 needs a pump having a high ability and thus uneconomical.
Next, an explanation will be made about the plasticizing cylinder 907 of the injection molding apparatus 4000. As shown in FIG. 15, the plasticizing cylinder 907 includes a screw 921 which is arranged rotatably and movably back and forth in the plasticizing cylinder 907, a shutoff valve 915 provided at the forward end of the screw 921, and an introduction opening 919 through which a resin pellet is introduced. An unillustrated mold is provided in close contact with the forward end of the plasticizing cylinder 907. Further, the plasticizing cylinder 907 includes a plasticizing and melting zone 925 in which the resin introduced from the introduction opening 919 is plasticized and melted, a physical foaming agent kneading zone 926 in which the plasticized and melted resin and the physical foaming agent are kneaded, and a zone for again raising pressure 927 in which the molten resin in which the physical foaming agent is dissolved is weighed or measured. The introducing valve 920 through which the physical foaming agent is introduced is provided in the physical foaming agent kneading zone 926. Further, there is provided a seal mechanism such as a halved seal ring 924, between the plasticizing and melting zone 925 and the physical foaming agent kneading zone 926, to prevent the high pressure physical foaming agent from flowing backward in a right-hand direction in FIG. 15.
The physical foaming agent introduced into the plasticizing cylinder 907 is kneaded with the resin and the foam injection molding is performed by, for example, a technique or approach described below. At first, the resin pellet supplied from the introduction opening 919 is plasticized and melted in the plasticizing and melting zone 925. Next, the physical foaming agent introduced from the introducing valve 920 is sheared by rotation of the screw to be compatibly dissolved with the molten resin uniformly in the physical foaming agent kneading zone 926. Subsequently, the molten resin in which the physical foaming agent is dissolved is fed to the front of the screw 921 and weighed, while adjusting the pressure by back pressure, in the zone for again raising pressure 927. The screw 921 moves backward in the right-hand direction in FIG. 15 upon the weighing, and stops at a weighing position to accumulate the molten resin of a predetermined amount at the front of the screw 921. Thereafter, the shutoff valve 915 is opened to move the screw forward (left-land direction in FIG. 15) while controlling velocity, and the resin in which the physical foaming agent is dissolved is injected and charged into the mold (not shown). The volume of the physical foaming agent, which is subjected to the sudden pressure reduction in the mold, expands in the molten resin to form foams (bubbles). By solidifying the resin, fine closed foams of about several microns to about 100 microns are formed at the inside of the molded product.
In the foam injection molding, the low molecular physical foaming agent having low viscosity is partially separated from the surface of the molten resin and gasified upon the injection and charging, and then discharged into the mold before the molten resin. Regarding the molten resin injected into the mold, at first, the resin at the forefront of resin flow, referred to as a flow front, is cooled on a wall surface of the mold to form a skin layer of the molded product. Although a part of the gasified physical foaming agent is redissolved in the skin layer of the molten resin, since the skin layer of the molded product is cooled and solidified instantly by being brought into contact with the surface of the mold, the skin layer has high viscosity and is less likely to redissolve the gas. Therefore, the gas which is not redissolved remains at a gap between the wall surface of the mold and the skin layer of the molded product to form a concavity on the surface of the molded product. The concavity on the surface is referred to as a swirl mark, which is a problem in terms of poor appearance of the foam molded product.
Meanwhile, the inventors of the present invention have suggested a method for producing a foam molded plating product, in which the metallic fine particles functioning as the catalyst cores of the plating are segregated on the surface of the molded product concurrently with the physical foaming to be capable of performing the plating without any chemical agents having heavy environmental burden (for example, Japanese Patent Application Laid-open No. 2007-130982). Also in this method, there is a problem that remains to be solved that the foam molded product is manufactured to have a smooth surface in which the catalyst cores of the plating are dispersed.
As a technique for making the surface of the foam molded product smooth to prevent the occurrence of the poor appearance, improvement of the mold has been suggested. For example, in Japanese Patent No. 3845191, it has been suggested a method for avoiding the swirl mark by performing heat insulation of the surface of the mold. In this method, by twilling the surface of the mold with a base material having low thermal conductivity to provide heat insulation of the surface of the mold, growth of the skin layer is suppressed. Further, by facilitating redissolution of the gas, the swirl mark is avoided.
As another improvement of the mold, it has been also suggested a technique in which temperature of the surface of the mold is made to be high upon the injection and charging to control the growth of the skin layer. In this technique, the swirl mark is redissolved into the skin layer to disappear, and thereafter the mold is cooled to solidify the molded product. Accordingly, it is possible to form the foam molded product having a foamed layer therein and a smooth surface. As a method for heating the surface of the mold, a method for flowing steam through a temperature control channel, a method for embedding a heater, a method for heating a cavity by an electromagnetic induction heating method, and the like have been investigated.
The method for forming the metal film on the molded product without the etching step as the pretreatment, described in each of Japanese Patent Application Laid-open No. 2005-280362, Japanese Patent Application Laid-open No. 2010-30106 and Japanese Patent No. 4160623, requires a dedicated molding machine. Therefore, in the method suggested in each of Japanese Patent Application Laid-open No. 2005-280362, Japanese Patent Application Laid-open No. 2010-30106 and Japanese Patent No. 4160623, it is not possible to form the molded product subjected to the electroless plating without the etching step having heavy environmental burden by using a general-purpose molding machine. It is considered that popularization of the method described in each of Japanese Patent Application Laid-open No. 2005-280362, Japanese Patent Application Laid-open No. 2010-30106 and Japanese Patent No. 4160623 is prevented due to an inability to use the general-purpose molding machine.
Further, in the method described in each of Japanese Patent Application Laid-open No. 2005-280362, Japanese Patent Application Laid-open No. 2010-30106 and Japanese Patent No. 4160623, the plating film grows toward the surface of the molded product from the inside of the molded product by using the palladium in the molded product as the catalyst. In this situation, in a case that the palladium is not unevenly distributed in the vicinity of the surface, a plating solution penetrates deeply in search of the catalyst and the plating film grows from a deep portion in the molded product. Accordingly, the resin is stretched at the inside of the molded product to occur brittle failure, thereby decreasing tight contact force of the plating film. Further, in a case that density of the palladium in the vicinity of the surface of the molded product is lowered, an uniform plating film can not be formed and thereby the poor appearance such as unevenness of the plating and a pinhole may be caused.
Also in Japanese Patent Application Laid-open No. 2007-130982, in order to obtain the surface of the molded product having the highly advanced function property, the functional material is preferably unevenly distributed on the surface of the molded product.
The present teaching solves the first problem described above and provides a method for producing a molded product subjected to electroless plating without an etching step having heavy environmental burden by using a general-purpose molding machine. Further, there is provided a method for producing a molded product including a functional material such as metallic fine particles, wherein the functional material is efficiently arranged in the vicinity of the surface of the molded product to promote surface modification of the molded product.
In the foam injection molding method disclosed in Japanese Patent No. 3845191, a heat-insulating layer is formed by a film having low strength such as ceramic or resin to prevent occurrence of the poor appearance of the molded product. Thus, durability of the mold is decreased. Further, in the technique of Japanese Patent No. 3845191, although the growth of the skin layer can be suppressed, it is difficult to uniformly control growth speed and/or viscosity of the skin layer over a large area. Thus, there is problem such that the scope of application of the technique of Japanese Patent No. 3845191 is narrow. Furthermore, the technique in which the temperature of the surface of the mold is made to be high upon the injection and charging requires initial investment for facilities, and also there is problem such that cost per mold is expensive.
Hence, the improvement of the mold is not adopted actually, and the poor appearance of the molded product is often modified by application of coating. Development of a technique and apparatus for making the surface of the foam molded product smooth to prevent the occurrence of the poor appearance is strongly expected, but there is no essential solution, which prevents popularization of a physical-foam injection molding method.
An object of the present teaching is to solve the second problem described above and to provide a physical-foam injection molding method for making a surface of a foam molded product smooth to suppress occurrence of poor appearance, a nozzle unit using the same, and an injection molding apparatus including the nozzle unit.